1. Field of the Invention
In general, the present invention relates to a method of fabrication of semiconductor processing apparatus from an aluminum substrate. In particular, the invention relates to a structure which provides a particular interface between an aluminum surface and aluminum oxide overlying that surface. The invention also relates to a method of producing the interfacial structure.
2. Brief Description of the Background Art
Semiconductor processing involves a number of different chemical and physical processes whereby minute integrated circuits are created on a substrate. Layers of materials which make up the integrated circuit are created by chemical vapor deposition, physical vapor deposition and epitaxial growth, for example. Some of the layers of material are patterned using photoresist masks and wet and dry etching techniques. Patterns are created within layers by the implantation of dopants at particular locations. The substrate upon which the integrated circuit is created may be silicon, gallium arsenide, indium phosphide, glass, or any other appropriate material.
Many of the semiconductor processes used to produce integrated circuits employ halogen or halogen-containing gases or plasmas. Some processes use halogen-containing liquids. In addition, since the processes used to create the integrated circuits leave contaminant deposits on the surfaces of the processing apparatus, such deposits are commonly removed using plasma cleaning techniques which employ at least one halogen-containing gas. The cleaning procedure may include a wet wipe with deionized water, followed by a wipe with isopropyl alcohol.
Aluminum has been widely used as a construction material for semiconductor fabrication equipment, at times because of its conductive properties, and generally because of its ease in fabrication and its availability at a reasonable price. However, aluminum is susceptible to reaction with halogens such as chlorine, fluorine, and bromine, to produce, for example, AlCl3; Al2Cl6; AlF3; or AlBr3. The aluminum-fluorine compounds can flake off the surfaces of process apparatus parts, causing an eroding away of the parts themselves, and serving as a source of particulate contamination of the process chamber (and parts produced in the chamber). Many of the compounds containing aluminum and chlorine and many of the compounds containing aluminum and bromine are volatile and produce gases under semiconductor processing conditions, which gases leave the aluminum substrate. This creates voids in the structure which render the structure unstable and produce a surface having questionable integrity.
A preferred means of protecting the aluminum surfaces within process apparatus has been an anodized alumina coating. Anodizing is typically an electrolytic oxidation process that produces an integral coating of relatively porous aluminum oxide on the aluminum surface. Despite the use of anodized alumina protective layers, the lifetime of anodized aluminum parts in semiconductor processing apparatus, such as susceptors in CVD reactor chambers, and gas distribution plates for etch process chambers has been limited, due to the gradual degradation of the protective anodized film. Failure of the protective anodized film leads to excessive particulate generation within the reactor chamber, requiring maintenance downtime for replacing the failed aluminum parts and for cleaning particulates from the rest of the chamber.
Miyashita et al., in U.S. Pat. No. 5,039,388, issued Aug. 13, 1991, describe a plasma forming electrode used in pairs in a semiconductor processing chamber. The electrode is formed from a high purity aluminum or an aluminum alloy having a chromic acid anodic film on the electrode surface. The chromic acid anodized surface is said to greatly improve durability when used in a plasma treatment process in the presence of fluorine-containing gas. The electrode is described as formed from a high purity aluminum such as JIS 1050, 1100, 3003, 5052, 5053, and 6061 or similar alloys such as Ag—Mg alloys containing 2 to 6% by weight magnesium.
U.S. Pat. No. 5,756,222 to Bercaw et al., issued May 26, 1998, and titled: “Corrosion-Resistant Aluminum Article For Semiconductor Processing Equipment”, describes an article of manufacture useful in semiconductor processing which includes a body formed from a high purity aluminum-magnesium alloy having a magnesium content of about 0.1% to about 1.5% by weight, either throughout the entire article or at least in the surface region which is to be rendered corrosion-resistant, and a mobile impurity atom content of less than 0.2% by weight. Mobile impurity atoms are said to consist of metal atoms other than magnesium, transitional metals, semiconductors, and atoms which form semiconductor compounds. Mobile impurity atoms particularly named include silicon, iron, copper, chromium and zinc. The high purity aluminum-magnesium alloy may be overlaid by a cohesive film which is permeable to fluorine, but substantially impermeable to oxygen. Examples of such a film include aluminum oxide or aluminum nitride. The subject matter disclosed in this patent is hereby incorporated by reference in its entirety.
U.S. Pat. No. 5,811,195 to Bercaw et al., issued Sep. 22, 1998, and titled: “Corrosion-Resistant Aluminum Article For Semiconductor Equipment”, further discloses that the magnesium content of the aluminum article may be in the range of about 0.1% to about 6.0% by weight of the aluminum article. However, for operational temperatures of the article which are greater than about 250° C., the magnesium content of the aluminum article should range between about 0.1% by weight and about 1.5% by weight of the article. In addition, an article is described in which the mobile impurities other than magnesium may be as high as about 2.0% by weight in particular instances. One example is when there is a film overlying the exterior region of the article body, where the film comprises aluminum oxide or aluminum. Another example is where there is a magnesium halide layer having a thickness of at least about 0.0025 microns over the exterior surface of the aluminum article. The subject matter disclosed in this patent is hereby incorporated by reference in its entirety.
For an aluminum alloy to be useful in the fabrication of semiconductor processing apparatus, it must not only exhibit the desired magnesium content and low level of mobile impurity atoms, but it must also have desirable mechanical properties. The mechanical properties must enable machining to provide an article having the desired dimensions. For example, if the alloy is too soft, it is difficult to drill a hole, as material tends to stick during the drilling rather than to be removed by the drill. Controlling the dimensions of the machined article is more difficult. There is a penalty in machining cost. In addition, the mechanical properties of the article affect the ability of the article to perform under vacuum. For example, a process chamber must exhibit sufficient structural rigidity and resistance to deformation that it can be properly sealed against high vacuum. Finally, the mobile impurities need to be uniformly distributed throughout the article so that there is a uniform transfer of loads and stresses.
The “Metals Handbook, Ninth Edition”, Volume 2, copyright 1979 by the American Society for Metals, describes the heat treatment of aluminum alloys beginning at Page 28. In particular, for both heat treatable and non-heat-treatable aluminum alloys, annealing to remove the effects of cold work is accomplished by heating within a temperature range from about 300° C. (for batch treatment) to about 450° C. (for continuous treatment). The term “heat treatment” applied to aluminum alloys is said to be frequently restricted to the specific operations employed to increase strength and hardness of the precipitation-hardenable wrought and cast alloys. These are referred to as “heat-treatable” alloys, to distinguish them from alloys in which no significant strengthening can be achieved by heating and cooling. The latter are generally said to be referred to as “non-heat-treatable” alloys, which, in wrought form, depend primarily on cold work to increase strength. At Page 29, Table 1 provides typical full annealing treatments for some common wrought aluminum alloys. The 5xxx series of alloys are considered to be “non-heat-treatable” aluminum alloys and are annealed at about 345° C. The 5xxx series of aluminum alloys are of interest for use in fabricating semiconductor processing apparatus because some of the alloys offer mobile impurity concentrations within acceptably moderate ranges, while providing sufficient magnesium content to perform in the manner described in the Bercaw et al. patents.
Standard thermal stress relief of “non-heat-treatable” aluminum alloys such as the 5xxx series assumes peak temperatures approaching 345° C. and generic ramp rates and dwell times, without regard to the alloy or the final use of individual articles fabricated from the alloy. Aluminum alloys begin to exhibit grain growth at temperatures approaching 345° C., and enhanced precipitation of non-aluminum metals at the grain boundaries, which may lead to cracking along the grain boundaries during machining. The above factors also reduce the mechanical properties of the alloy, by affecting the uniformity of the alloy composition within the article.
When the article fabricated from an aluminum alloy is to be used in a corrosive atmosphere, it frequently necessary to provide a protective coating such as anodized aluminum over the aluminum surface. This is particularly true for applications of aluminum in semiconductor processing where corrosive chlorine or fluorine-containing etchant gases and plasmas generated from these gases are employed. A stable aluminum oxide layer over the aluminum alloy surface can provide chemical stability and physical integrity which is effective in protecting the aluminum alloy surface from undergoing progressive erosion/corrosion. As described in the Bercaw et al. patents, the presence of an aluminum oxide layer over the surface of the specialty magnesium-containing aluminum alloy described therein helps maintain a magnesium halide protective component at or near the surface of the aluminum alloy. The aluminum oxide helps prevent abrasion of the relatively soft magnesium halide component. The combination of the aluminum oxide film and the magnesium halide protective component overlying the specialty aluminum alloy provides an article capable of long term functionality in the corrosive environment. However, one requirement which has not been adequately addressed in the past is the mechanical performance of the article. In attempting to obtain the mechanical properties required for the aluminum alloy body of the article, it is possible to affect the surface of the aluminum alloy in a manner such that a subsequently-formed aluminum oxide (anodized) layer does not form a proper interface with the aluminum alloy, especially at the grain boundary areas. This creates gaps between the aluminum oxide layer and the underlying aluminum surface. This porosity promotes a breakdown in the protective aluminum oxide layer, which leads to particle formation, and may cause a constantly accelerating degradation of the protective aluminum oxide film.
Not only is there significant expense in equipment maintenance and apparatus replacement costs due to degradation of the protective aluminum oxide film, but if a susceptor, for example, develops significant surface defects, these defects can translate through a silicon wafer atop the susceptor, creating device current leakage or even short. The loss of all the devices on a wafer can be at a cost as high as $50,000 to $60,000 or more.
It is clear that there are significant advantages to providing an interface between a protective aluminum oxide and the underlying aluminum alloy with sufficient stable mechanical, chemical, and physical properties to extend the lifetime of the protective film. It is also clear that it would be beneficial to provide a less porous, dense, and more stable aluminum oxide film.